Handle payment response

When you call show() method on a PaymentRequest object, it returns the
payment request response. After the response is received, it must be sent back
to the retailer server. There, the transaction ID is verified against
PSP to ensure the transaction was successful. The UI is then updated for the
user based on the verification status.

To aid in verification, the response from Google Pay contains the fields required by
NPCI. For more information, see the NPCI UPI Linking Specification.
Additionally, Google Pay provides the flexibility of a customized response.
Refer to Google Pay response
section for detailed response fields.

The following example shows how the browser response is converted to a JSON
string and sent back to the retailer server, then further processed. After
receiving the server response, it will complete the payment by logging a message
in the console.